The present invention relates to a thermal storage device for maintaining the temperature of an article at a desired temperature for a period of time. More particularly, the invention relates to such a device which comprises a sorption compression refrigeration system that, when activated by an external power supply, generates a quantity of pressurized refrigerant that may later be controllably evaporated to produce a cooling effect and thereby maintain the temperature of the article at the desired temperature for a period of time.
The present invention is particularly useful as a shipping container for refrigerated articles, such as frozen foods. Frozen foods must usually be shipped in refrigerated trucks or individual shipping containers which are packed with ice. However, refrigerated trucks are generally not well insulated and therefore require one or more relatively large vapor compression refrigeration units to maintain the temperature of the cargo at a desired temperature. These refrigeration units are typically powered by the electrical system of the truck; consequently, they can significantly reduce the fuel efficiency of the truck. In addition, the use of a refrigerated truck is not economical when less than an entire truckload of articles is to be shipped. On the other hand, ice-packed shipping containers do not allow for precise temperature control, require a fresh source of ice each time they are used, and must be packed and shipped before the ice melts. Consequently, these types of shipping containers are usually only practical when shipping certain types of articles relatively short distances.
In accordance with the present invention, these and other limitations in the prior art are overcome by providing a thermal storage device for maintaining the temperature of an article at a desired temperature for a length of time. The thermal storage device comprises a compartment within which the article may be positioned, an evaporator which is disposed in heat exchange relation with respect to the compartment, a receiver which is fluidly connected to the evaporator, a sorber which is fluidly connected between the evaporator and the receiver and which includes a sorbent that is capable of adsorbing a refrigerant, means for desorbing the refrigerant from the sorbent, and means for releasably connecting an external power supply to the desorbing means. When the desorbing means is connected to the external power supply, the refrigerant is desorbed from the sorbent and communicated to the receiver. Furthermore, after the desorbing means is disconnected from the external power supply, the refrigerant within the receiver may be evaporated in the evaporator and adsorbed onto the sorbent to thereby produce a cooling effect in the compartment. Thus, the thermal storage device is capable of cooling the compartment after it has been disconnected from the external power supply.
In a preferred embodiment of the invention, the desorbing means comprises first and second electrical conductors between which the sorbent is positioned, and the connecting means includes a pair of electrical leads which are electrically connected to the first and second conductors. In addition, the sorbent and the refrigerant are selected such that, when an electrical current is conducted through the sorbent, the current will desorb the refrigerant from the sorbent. In this manner, the refrigerant may be desorbed from the sorbent by connecting the leads to the external power supply, which can be a conventional source of line voltage. This provides a convenient means for xe2x80x9cchargingxe2x80x9d the thermal storage device prior to use.
In one embodiment of the invention, the thermal storage device includes means for controlling the flow of refrigerant into the evaporator. Such means could be, for example, an orifice valve, a capillary tube, or a manual, electrical or pressure actuated valve. Thus, when the temperature of the article rises above the desired temperature, the flow control means is operable to allow the refrigerant to evaporate and thereby cool the article.
In another embodiment of the invention, the thermal storage device comprises a valve which is fluidly connected between the receiver and the evaporator, a temperature sensor which is thermally connected to the compartment, and means for indicating whether the temperature of the compartment is above the desired temperature. Thus, when the temperature of the compartment rises above the desired temperature, the valve may be opened to allow the refrigerant to evaporate and thereby cool the compartment.
In yet another embodiment of the invention, the thermal storage device includes a controllable valve which is fluidly connected between the receiver and the evaporator, a temperature sensor which is thermally connected to the compartment, and a controller which is connected to both the temperature sensor and the valve. Thus, when the temperature of the compartment rises above the desired temperature, the controller will open the valve to allow the refrigerant to evaporate and thereby cool the compartment. More preferably, the controller also operates to close the valve when the temperature of the compartment drops below the desired temperature. In this manner, the thermal storage device can automatically maintain the temperature of the compartment, and thus the article, at the desired temperature for a length of time.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar elements in the various embodiments.